In an optical communication system or the like, optical circulators and optical switches are used as optical devices for optical path control. The optical circulator is an optical device having a light separating function for outputting an input light from a certain port to only another particular port, e.g. outputting an input light from a first port P1 to a second port P2 and outputting an input light from a second port P2 to a third port P3, thereby forming a cyclic optical path. Meanwhile, an optical switch is an optical device having an optical-path switching function of outputting an input light from a first port P1 to either of a third port P3 or a fourth port P4 and outputting an input light from a second port P2 to either of a fourth port P4 or a third port P3.
The optical circulator uses a 45-degree Faraday rotator for applying a fixed magnetic field by a permanent magnet, to rotate a polarization plane by 45 degrees toward a predetermined direction thereby realizing the reciprocality of rays. The optical switch uses a variable Faraday rotator that changes an application magnetic field direction by an electromagnet, thereby realizing optical-path switching. However, the basic part can be structured nearly similarly.
Conventionally, various structures of optical circulators and optical switches have been developed. As one example of those, there is provided a structure wherein polarized-light separating/combining elements having polarized-light separating/combining films are arranged with spacing to insert optical rotators, made by a Faraday rotator and a ½-wavelength plate, therebetween. The conventional polarized-light separating/combining element, generally, is a polarizing beam splitter (PBS) in a structure having two glass-made triangular prisms bonded together through a sandwiched polarized-light separating/combining film (dielectric multi-layer film). There is also a polarized-light separating/combining element having two surfaces of polarized-light separating/combining films. In such a case, triangular prisms are bonded on the both ends of a prism having a parallelogram section through polarized-light separating/combining films (dielectric multi-layer films) into a rectangular structure.
In the conventional polarized-light separating/combining element, it is ideal to output light parallel with the input light. However, the output light actually has a slight angle with respect to the input light. Although the angular deviation results from dimensional and angular errors on each prism and prism bonding accuracy, there is a defect of beam coupling loss increase.
Meanwhile, the conventional polarized-light separating/combining element, because of its low extinction ratio of polarized light, has a problem that a crosstalk characteristic is poor when built in the optical device.